Protein kinases regulate nearly every cellular process, including metabolism, cell proliferation, cell differentiation, and cell survival, so they are attractive targets for therapeutic intervention for various disease states. For example, cell-cycle control and angiogenesis, in which protein kinases play a pivotal role are cellular processes associated with numerous disease conditions such as but not limited to cancer, inflammatory diseases, abnormal angiogenesis and diseases related thereto, atherosclerosis, macular degeneration, diabetes, obesity, and pain.
One of the principal mechanisms by which cellular regulation is effected is through the transduction of extracellular signals across the membrane that in turn modulate biochemical pathways within the cell. Protein phosphorylation represents one course by which intracellular signals are propagated from molecule to molecule resulting finally in a cellular response. These signal transduction cascades are highly regulated and often overlap, as is evident from the existence of many protein kinases as well as phosphatases. Phosphorylation of proteins occurs predominantly at serine, threonine or tyrosine residues, and protein kinases have therefore been classified by their specificity of phosphorylation site, i.e. serine/threonine kinases and tyrosine kinases. Since phosphorylation is such a ubiquitous process within cells and since cellular phenotypes are largely influenced by the activity of these pathways, it is currently believed that a number of disease states and/or diseases are attributable to either aberrant activation or functional mutations in the molecular components of kinase cascades. Consequently, considerable attention has been devoted to the characterisation of these proteins and compounds that are able to modulate their activity (for a review see: Weinstein-Oppenheimer et al. Pharma. &. Therap., 2000, 88, 229-279).
IKKε and TBK1 are serine/threonine kinases which are highly homologous to one another and other IkB kinases. The two kinases play an integral role in the innate immune system. Double-stranded RNA viruses are recognised by the Toll-like receptors 3 and 4 and the RNA helicases RIG-I and MDA-5 and result in activation of the TRIF-TBK1/IKKe-IRF3 signalling cascade, which results in a type I interferon response.
In 2007, Boehm et al. described IKKε as a novel breast cancer oncogene (J. S. Boehm et al., Cell 129, 1065-1079, 2007). 354 kinases were investigated with respect to their ability to recapitulate the Ras-transforming phenotype together with an activated form of the MAPK kinase Mek. IKKε was identified here as a cooperative oncogene. In addition, the authors were able to show that IKKε is amplified and overexpressed in numerous breast cancer cell lines and tumour samples. The reduction in gene expression by means of RNA interference in breast cancer cells induces apoptosis and impairs the proliferation thereof. Eddy et al. obtained similar findings in 2005, which underlines the importance of IKKε in breast cancer diseases (S. F. Eddy et al., Cancer Res. 2005; 65 (24), 11375-11383).
A protumorigenic effect of TBK1 was reported for the first time in 2006. In a screening of a gene library comprising 251,000 cDNA, Korherr et al. identified precisely three genes, TRIF, TBK1 and IRF3, which are typically involved in the innate immune defense as proangiogenic factors (C. Korherr et al., PNAS, 103, 4240-4245, 2006). In 2006, Chien et al. (Y. Chien et al., Cell 127, 157-170, 2006) published that TBK1−/− cells can only be transformed to a limited extent using oncogenic Ras, which suggests an involvement of TBK1 in the Ras-mediated transformation. Furthermore, they were able to show that an RNAi-mediated knock-down of TBK1 triggers apoptosis in MCF-7 and Panc-1 cells. Barbie et al. recently published that TBK1 is of essential importance in numerous cancer cell lines with mutated K-Ras, which suggests that TBK1 intervention could be of therapeutic importance in corresponding tumours (D. A. Barbie et al., Nature Letters 1-5, 2009).
Diseases caused by protein kinases are characterised by anomalous activity or hyperactivity of such protein kinases. Anomalous activity relates to either: (1) expression in cells which do not usually express these protein kinases; (2) increased kinase expression, which results in undesired cell proliferation, such as cancer; (3) increased kinase activity, which results in undesired cell proliferation, such as cancer, and/or in hyperactivity of the corresponding protein kinases. Hyperactivity relates either to amplification of the gene which encodes for a certain protein kinase, or the generation of an activity level which can be correlated with a cell proliferation disease (i.e. the severity of one or more symptoms of the cell proliferation disease increases with increasing kinase level). The bioavailability of a protein kinase may also be influenced by the presence or absence of a set of binding proteins of this kinase.
IKKε and TBK1 are highly homologous Ser/Thr kinases critically involved in the innate immune response through induction of type 1 interferons and other cytokines. These kinases are stimulated in response to viral/bacterial infection. Immune response to viral and bacterial infection involves the binding of antigens such as bacterial lipopolysaccharide (LPS), viral doublestranded RNS (dsRNA) to Toll like receptors, then subsequent activation of TBK1 pathway. Activated TBK1 and IKKε phosphorylate IRF3 and IRF7, which triggers the dimerization and nuclear translocation of those interferon regulatory transcription factors, ultimately inducing a signaling cascades leading to IFN production.